The invention relates to a pump arrangement for medical purposes, in particular a blood or infusion pump, comprising at least one pump housing having a chamber which is divided by an impermeable diaphragm into a working chamber and a drive chamber, the working chamber being provided with at least one opening for the supply and discharge of the fluid to be conveyed and the drive chamber being adapted to be subjected to air as pumping fluid and connected to a pump having a limited pumping volume, and a valve connected to the drive chamber for pressure compensation on stroke change of the pump.
A pump arrangement of the type mentioned at the beginning is known from DE-OS No. 3,205,449 (corresponding to U.S. patent application Ser. No. 466,645 filed Feb. 15, 1983). It consists substantially of a reservoir vessel which is divided into two chambers. The one chamber can be filled and emptied with the fluid to be pumped, in particular blood, whilst the other chamber alternately thereto is subjected to air as pumping fluid. Between these two chambers a resiliently extensible impermeable diaphragm is provided which according to this German specification as laid open to inspection is in the form of a resilient flexible tube. The two chambers are surrounded by a rigid air-impermeable pump housing which for the chamber containing the pumping fluid has an opening which is connected via a conduit to the pump having a limited pumping volume. This pump is constructed as bellows pump which in pumping operation oscillates between the upper and lower dead-centre.
The chamber adapted to be subjected to the pumping fluid or the space containing the pumping fluid is connected via vacuum and pressure control valves to the surroundings. The purpose of these valves is to maintain a certain pressure value, that is a certain excess pressure and reduce pressure in the operation of the pump. If for example the pump is blocked or the usual starting difficulties occur only a supplying or discharge of the pumping fluid takes place until the pressure value controlled by the valves is achieved, which usually operate automatically.
However, usually pumping operation takes place in such a manner that the excess pressure and reduced pressure control valves are not actuated. In such a mode of operation the pumping fluid is thus compressed on discharge of the pumping chamber (excess pressure) and expanded (reduced pressure) on filling of the pumping chamber. However, when operating the pump arrangement problems are encountered which in particular affect the pumping capacity of this arrangement.
Below, the mode of operation of the known pumps and the problems involved will be explained.
As already mentioned the pressure in the pump fluctuates between a lower pressure value p.sub.min and an upper pressure value p.sub.max at which the valves are just on the point of opening.
Now, depending on the use of the pump a predetermined suction pressure p.sub.E and delivery pressure p.sub.K is required which are to be applied to the pump arrangement.
Assuming that for the diaphragm no force is required for the expansion then in the limit case p.sub.E =p.sub.min or p.sub.K =p.sub.max.
The pumping behavior of the known pump will now be explained with reference to the following example:
p.sub.E =-400 mbar and p.sub.K =+600 mbar whilst the internal volume of the pumping chamber V.sub.1 =50 cm.sup.3 and the bellows volume of the bellows pump V.sub.2 =30 cm.sup.3.
In the initial state V.sub.1 (compressed)=0 cm.sup.3 and V.sub.2 =30 cm.sup.3 whilst p=p.sub.max =+600 mbar absolute=1600 mbar.
Subsequently, suction takes place and V.sub.1 approaches 50 cm.sup.3 and V.sub.2 approaches 0 cm.sup.3. The volume is thus expanded from 30 to 50 cm.sup.3.
From the relationship p.times.V=a constant there follows:
1600.times.30=p.sub.E .times.50 PA1 p.sub.E =960 mbar. PA1 1600.times.30=600.times.V.sub.T (V.sub.T =necessary bellows volume) PA1 V.sub.T =80 cm.sup.3 PA1 V.sub.1 =0 cm.sup.3 PA1 V.sub.2 =30 cm.sup.3 PA1 p=p abs.=1000 mbar. PA1 p.sub.E 30.times.1000=50.times.p.sub.E PA1 p.sub.E =600 mbar abs.=-400 mbar rel. PA1 V.sub.1 =50 cm.sup.3 PA1 V.sub.2 =0 PA1 p=1000 mbar PA1 1000.times.50=p.sub.K .times.30 PA1 p.sub.K =1665 mbar abs., i.e. 665 mbar rel.
As a result the expansion reduced pressure p.sub.E is only -40 mbar abs., and this is not adequate for operating the pump.
If now on expansion of the bellows pump -400 mbar is to be attained as explained below a lower pumping volume is merely obtained. For it follows from the above relationship that:
i.e. in the limit case although the suction reduced pressure or vacuum can be reached the bellows volume of 50 cm.sup.3 is not adequate for a complete discharge. For this purpose 80 cm.sup.3 is required.
A further pump arrangement is known from U.S. Pat. No. 3,568,214 in which two pumping chambers are alternately subjected to pumping fluid from a pump, isotomic saline solution or mercury being used as pumping fluid. The displacement of the phase position on stroke change of the pump occurring in such systems considerably affects the action of the pump and usually cannot be controlled so that this pump has proved impracticable.
The invention is therefore based on the problem of providing a pump arrangement of the type mentioned at the beginning which within the set pressure limits in operation completely fills and empties the pumping chamber.